For example, a high intensity discharge lamp such as a xenon lamp and an extra-high pressure mercury lamp has been used in a projector for image display such as a DLP (TM) projector and a liquid crystal projector. As an example, the principle of such a projector is shown in FIG. 10 (reference: Japanese Patent Application Publication No. 2004-252112 etc.).
As described above, light from a light source (UsA), which comprises a high intensity discharge lamp, is inputted into an incident end (PmiA) of a light uniformizing unit (FmA) by, for example, using a condensing unit (not shown), which consists of a concave reflection mirror, a lens, etc., and is outputted from an emission end (PmoA). Here, for example, an optical guide can be used as the light uniformizing unit (FmA), which is also called a rod integrator, a light tunnel, etc., and is formed of a prism, which is made from light transmissive material such as glass, resin, etc., wherein while the light inputted into the incident end (PmiA) is repeatedly and totally reflected on a side face of the light uniformizing unit (FmA) according to the principle, which is the same as that of an optical fiber, it propagates inside the light uniformizing unit (FmA), thereby functioning so that the illuminance on the emission end (PmoA) is sufficiently uniformized even if distribution of the light inputted into the incident end (PmiA) has unevenness.
An illumination lens (Ej1A) is arranged so that a quadrangle image of the emission end (PmoA) is formed on a two-dimensional light amplitude modulation element (DmjA), whereby a two-dimensional light amplitude modulation element (DmjA) is illuminated by light outputted from the emission end (PmoA). However, in FIG. 10, a mirror (MjA) is arranged between the illumination lens (Ej1A) and the two-dimensional light amplitude modulation element (DmjA). And the two-dimensional light amplitude modulation element (DmjA) modulates light on a pixel to pixel basis according to an image signal so that the light may be directed so as to enter the projection lens (Ej2A), or light may be directed so as not to enter there, whereby an image is displayed on a screen (Tj).
Since such a two-dimensional light amplitude modulation element, which is described above is also called a light valve, and in the case of the optical system shown in FIG. 10, in general, a DMD (TM) (Digital Micromirror Device) is often used as the two-dimensional light amplitude modulation element (DmjA).
On the other hand, in case where a LCOS (TM) (Liquid Crystal on Silicon), which is a silicon liquid crystal device, is used as a two-dimensional light amplitude modulation elements, a principle diagram thereof is shown in an FIG. 11 as an example of a projector using it (reference: Japanese Patent Application No. 2001-142141 etc.).
Light from a light source (UsB), which comprises a high intensity discharge lamp etc., is inputted, as approximately parallel light flux, into an incident end (PmiB) of a light uniformizing unit (FmB), which is called a fly eye integrator, by using a collimator unit (not shown), which is made up of a concave reflection mirror, a lens, etc., and is outputted from an emission end (PmoB). Here, the light uniformizing unit (FmB) is made up of a combination of an upstream fly eye lens (F1B) on an incident side, a downstream fly eye lens (F2B) on a light emission side, and an illumination lens (Ej1B). The upstream fly eye lens (F1B) and the downstream fly eye lens (F2B) are respectively formed by arranging, in vertical and horizontal directions, many quadrangle lenses whose focal distance is the same as one another and whose shape is the same as one another.
When, based on a focal distance of the upstream fly eye lens (F1B), the downstream fly eye lens (F2B), and the illumination lens (Ej1B), a two-dimensional light amplitude modulation element (DmjB) is arranged at a predetermined position derived from the constitutive theory of a fly eye integrator optical system, the two-dimensional light amplitude modulation element (DmjB) which is an object to be illuminated, is illuminated by the light outputted from the emission end (PmoB). However, the light is reflected towards the two-dimensional light amplitude modulation element (DmjB) in the case of illumination, by arranging a polarization beam splitter (MjB) between the illumination lens (Ej1B) and the two-dimensional light amplitude modulation element (DmjB). And the two-dimensional light amplitude modulation element (DmjB) performs modulation and reflection so as to or so as not to rotate the polarization direction of light by 90 degrees on a pixel to pixel basis according to an image signal, whereby only the rotated light passes through the polarization beam splitter (MjB), and enters the projection lens (Ej3B), so that an image may be displayed on a screen (Tj).
In case of such a liquid crystal device, since only a component of light in a specified polarization direction can be modulated effectively, although a component parallel to the specified polarization direction is usually passed therethrough as it is, only a component perpendicular to the specified polarization direction is rotated by 90 degrees with respect to the polarization direction, so that a polarized-light alignment functional device (PcB) for making all the light effectively usable, is inserted, for example, downstream of the downstream fly eye lens (F2B). Moreover, a field lens (Ej2B) is inserted immediately upstream of the two-dimensional light amplitude modulation element (DmjB) so that approximately parallel light may enter the two-dimensional light amplitude modulation element (DmjB).
In addition to the reflection type of the two-dimensional light amplitude modulation element shown in FIG. 11, a transmissive liquid crystal device (LCD) may be used as the two-dimensional light amplitude modulation element in the optical arrangement which is suitable therefor (reference: Japanese Patent Application Publication No. H10-133303 etc.).
In general, for example, a dynamic color filter such as a color wheel is arranged downstream or upstream side of the light uniformizing unit in a projector in order to display a color image, and the two-dimensional light amplitude modulation element is illuminated with color sequential light flux of R, G and B (Red, Green, Blue), whereby color display is realized in a time dividing manner, or a dichroic mirror or a dichroic prism is arranged downstream of the light uniformizing unit, so that the two-dimensional light amplitude modulation element, which is independently provided in each color, is illuminated with light which is separated into the three primary colors of R, G and B, and a dichroic mirror or a dichroic prism for performing color synthesis of the modulated light flux of the primary colors R, G and B is arranged. However, for ease of explanation, in FIGS. 10 and 11, these elements are omitted.
A discharge lamp lighting apparatus for lighting the above-described discharge lamp is operated so that, at time of initiation thereof, while voltage called no-load open circuit voltage is applied to the lamp, high voltage is impressed to the lamp, so as to generate dielectric breakdown in an electrical discharge space, whereby the discharge state changes from glow discharge to arc discharge and finally a steady lighting state may be realized.
The discharge voltage of the lamp, which is low, for example, about 10 V, immediately after shifting to the arc discharge, gradually increases as the temperature rises, and becomes stable at a fixed voltage in a steady lighting state. In general, such a discharge lamp lighting apparatus has a converter for adjusting an output of an input power source to the discharge voltage of the lamp, so that an output lamp current required in order to realize a predetermined lamp input power may be outputted. Moreover, the discharge lamp lighting apparatus has mechanism for detecting lamp voltage, i.e., the output voltage of the converter, and determining target lamp current based on this information, for example, a value of the quotient obtained by dividing the target electric power by the detected voltage.
The discharge voltage of the lamp in a steady lighting state, i.e., lamp voltage, (VL) has a character in which it become lower as a distance between tips of two electric discharge electrodes, i.e., a distance between electrodes, become short. However, in the case, while the utilization efficiency of the light emitted from the lamp becomes high since it becomes closer to a point light source as the distance between the electrodes becomes shorter, where the same electric power is supplied to the lamp since the lamp voltage (VL) becomes low, there is a disadvantageous aspect in that heat generation of the discharge lamp lighting apparatus becomes large since lamp current (IL) becomes large. On the contrary, while the point light source nature is deteriorated so that the utilization efficiency of light becomes low as the distance between electrodes becomes long, where the same electric power is supplied to the lamp since lamp voltage (VL) becomes high, there is an advantageous aspect in that heat generation of discharge lamp lighting apparatus becomes small since the lamp current (IL) can be small. Therefore, it is not necessarily advantageous or advantageous that the distance between the electrodes is short, and it can be understood that the distance between the electrodes should be maintained between the upper and lower limits, which are specified from the brightness required as a light source for a projector, and the limit of heat generation of the discharge lamp lighting apparatus which can be processed, that is, a predetermined range.
As to types of a driving method of the discharge lamp, there are a direct current driving method in which the lamp is lighted by the converter, and an alternating current driving method in which repeated polarity reversals are performed by further providing an inverter downstream of the converter. In the case of the direct current driving method, since the light flux from the lamp is like direct current, that is, it does not change with passage of time, basically, there is a big advantage that it can be similarly applied to both types of the above-described projectors.
On the other hand, while in the case of alternating current drive system, while there is also a disadvantageous aspect resulting from existence of polarity reversals itself, such as a bad influence on a display image due to instantaneous turning-off and overshooting of light emitted at time of polarity reversals, in the case of the alternating current driving method, development or wear of the electrode(s) of the discharge lamp can be controlled by using the flexibility which does not exist in the direct current driving method such as polarity-reversal frequency.
Japanese Patent Application Publication No. 2001-312997 discloses prior art in which wear-out or growth of electrodes of a discharge lamp is controlled by controlling polarity-reversals frequency etc., and a distance between electrodes is maintained in a desired range, wherein the frequency is set to a first frequency when a distance between electrodes decreases from a normal value due to formation of a projection(s) on a portion(s) where the tips of electrodes of a high pressure discharge lamp face each other, and the frequency is set to a second frequency when the projection(s) decreases so that the distance between the electrodes increases from the normal value.
Moreover, Japanese Patent Application Publication No. 2002-175890 teaches that in alternating current drive of the lamp which has current-proof of regulation of the electrode, a period of 1 second or more where it becomes 5 Hz or less or a period of 1 second or more where lighting current becomes a rated current value or more, is generated.
Further for example, in Japanese Patent Application Publication No. 2003-133091 teaches that when the voltage between electrodes drops below a predetermined value due to a change of a distance between electrodes during lighting, a period where current at frequency lower than the rated frequency is temporarily prepared.
Further for example, Japanese Patent Application Publication No. 2003-338394 teaches that in case of lighting at power lower than the rated power, when voltage between electrodes drops below a predetermined value due to a change of a distance between electrodes, a period, where alternating current at frequency higher than that of lighting current at time of the rated power lighting is supplied, is set for a predetermined period.
Further, for example, Japanese Patent Application Publication No. 2004-342465 teaches that a full bridged circuit is operated in a polarity-reversal operation at alternating frequency, at which a projection(s) of the electrodes tends to be formed, for a definite period immediately after starting an electric discharge lamp, and a polarity-reversal operation is performed at alternating frequency, at which a change to electrodes is small, after passage of the definite period, or when tube voltage of an electric discharge lamp rises, alternating frequency is raised according to it, or a time ratio of positive and negative side in polarity reversals is changed according to the conditions thereof.
Further, for example, Japanese Patent Application No. 2005-197181 teaches that according to the magnitude relation of lamp voltage and switching voltage, polarity-reversal frequency is changed in multiple steps, wherein the frequency is fixed at a predetermined frequency and the lamp is lighted for a predetermined period from the starting time.
Further for example, Japanese Patent Application Publication No. 2006-140016 teaches that the frequency of alternating current is changed in a regular or irregular manner.
Further, for example, Japanese Patent Application Publication No. 2006-156414 teaches that bridge drive frequency is switched and controlled at two or more steps at time of lighting.
Further, for example, Japanese Patent Application Publication No. 2006-185663 teaches that the polarity-reversal frequency of a bridge is changed according to lamp voltage.
Further, for example, Japanese Patent Application Publication No. 2007-087637 teaches that a discharge lamp is lighted while low frequency is inserted when lighting voltage of the lamp is a first predetermined value or more, and low frequency is not inserted when the lighting voltage of the discharge lamp is a second predetermined value or less.
Of course, in many of the technologies, not all can be uniformly applied thereto even in case of an alternating current drive type lamp. That is, some are applicable, and some are not applicable, depending on a lamp design parameters such as electrode structure, dimension thereof, material structure, the constituent and contained amount of an electric discharge medium, form, a size, etc. of a bulb. Even in the applicable case, the range of polarity-reversal frequency, and the value of current must be strictly specified according to the design parameter of each lamp.
In view of the above, it can be understood that in the case of the alternating current drive type, in order to maximize the performance of a lamp and to perform a long life span operation thereof, it is very important to determine the proper polarity-reversal frequency according to the conditions of the lamp at each point of time. However, among the technologies described above, it may possible to use the technology in which the polarity-reversal frequency is gradually changed in a continuous or discontinuous way in order to merely light the lamp. However, the technology cannot be used in order to light the lamp as a light source for a projector.
The reason is as set forth below. Renewal of the state of a pixel(s) is repeated for a short period of time in the two-dimensional light amplitude modulation element (DmjA, DmjB) of a projector, and if updating timing of the pixel and polarity-reversal timing of the lamp is not adjusted for each other, as described above, instantaneous turning-off, overshooting etc. of light emission at time of polarity reversal, arise, thereby causing a bad influence on a display image. Therefore, the two-dimensional light amplitude modulation element driving circuit which drives a two-dimensional light amplitude modulation element (DmjA, DmjB), and a discharge lamp lighting apparatus which drives a lamp, are not individually operated, but it is necessary to perform an operation in cooperation so that the updating timing of a pixel(s) and the polarity-reversals timing of the lamp may be adjusted for each other, so that the synchronized signal for a synchronization is sent towards the discharge lamp lighting apparatus from the two-dimensional light amplitude modulation element driving circuit. Therefore, what can usually be performed in discharge lamp lighting apparatus, is that either polarity reversals are performed according to a synchronized signal, or a synchronized signal is ignored so that the polarity reversals may not be performed, so that without considering the convenience in a two-dimensional light amplitude modulation element driving circuit, based on a matter of convenience in the lamp, the polarity-reversal frequency cannot be gradually changed either in a continuous way or in a discontinuous way.
However, in recent years, in order to further realize a highly efficient and highly functional projector, in an operation of the two-dimensional light amplitude modulation element (DmjA, DmjB) a black screen, i.e., a momentary black screen may be inserted. In addition, a situation, in which the length of a black screen period is not constant, arises. The black screen is inserted in order to smooth a motion of an animation, or to prevent appearance of an imperfect image in a switching time of liquid crystal shutter spectacles, by inserting it between a right eye image and a left eye image in 3D display, wherein as to the length of a black screen insertion period, a variation is given continuously or gradually for a reason that it is adjusted for the switching speed performance etc. of the liquid-crystal shutter spectacles.
This situation is briefly described below, referring to FIGS. 12A and 12B. As shown in FIGS. 12A and 12B, when a black screen period (Pb) is changed from a short one to long one, if the length of one cycle is unchanged, the length of an effective screen period (Pw) other than the black screen period (Pb) becomes short. When the two-dimensional light amplitude modulation element driving circuit generates a polarity reversal notification signal (Sy) at timings (f0, f1, f2, f3, f0′, . . . ), even if an interval of the polarity reversal notification signal (Sy) generated at timings (f3, f0′) corresponding to a start and an end of the black screen period (Pb) is prolonged that shown in FIG. 12A to FIG. 12B, in general, It is difficult to expect to make a two-dimensional light amplitude modulation element driving circuit have a function for appropriately inserting the polarity reversal notification signal (Sy) in the middle thereof.
The reason thereof will be set forth below. As described above, since, in order to maximize the performance of the lamp and to perform a long life span operation thereof, it is necessary to determine the proper polarity-reversal frequency according to the conditions of the lamp at each point of time. Therefore, to appropriately determine whether a polarity-reversal operation should be inserted at the timing (f4) during the period by making a judgment from the length of the interval of the timings (f3, f0′), whether to insert it once, or whether to insert it twice determines correctly whether to insert in two places, is dependent on the design parameter of a lamp to be actually used, so that it is only possible to accomplish it by the discharge lamp lighting apparatus which is designed by maker who design a lamp. It is possible to determine, in advance, the polarity reversal notification signal (Sy) within the effective screen period (Pw) which is generated at timings (f1, f2), based on polarity-reversals frequency of the lamp to be used, that is, a proper value of the polarity-reversal interval which is a reciprocal index, by programming a two-dimensional light amplitude modulation element driving circuit according to a processing sequence performed in that period.